1. Field of the Invention
The present invention relates to a zoom lens system, particularly to an image-reading zoom lens system for use as a high-definition finite-conjugate-distance zoom lens system in an image copying apparatus, image reading apparatus, or the like.
2. Description of the Prior Art
Conventionally, a lens system designed for use in an image copying or reading apparatus is typically built as a fixed-focal-length lens system that offers fixed reading magnification. Even if such a lens system is designed to offer variable reading magnification, the range of magnification it offers is usually very limited. Customarily, reading magnification is varied either electrically or optically. For example, Japanese Laid-open Patent Applications Nos. H6-94993 and S57-73715 propose varying reading magnification by the use of a variable-focus lens.
On the other hand, increasing attention has been paid to a technique that allows proper correction of chromatic and other aberrations by combining a diffractive optical element with a refractive optical element. For example, Japanese Laid-open Patent Application No. H6-242373 proposes an objective lens system based on this technique for use in an optical disk apparatus. Moreover, U.S. Pat. No. 5,268,790 proposes applying the technique to a lens system for use in a video apparatus, and Japanese Laid-open Patent Application No. H4-214516 proposes applying the technique to a lens system for use in a stepper.
However, the variable-magnification lens system proposed in Japanese Laid-open Patent Application No. H6-94993 mentioned above requires a large variation in the conjugate distance, and thus cannot be made practically compact. On the other hand, the zoom lens system proposed in Japanese Laid-open Patent Application No. S57-73715 mentioned above allows the conjugate distance to be fixed, but is unduly large as a whole, and in addition suffers from too large aberrations such as distortion to be used for a high-definition image reading.
The objective lens system proposed in Japanese Laid-open Patent Application No. H6-242373 for use in an optical disk apparatus is designed for a narrow wavelength range. Thus, if used in an optical system that employs a halogen lamp, this lens system suffers from chromatic aberration. Moreover, the lens system for use in a video apparatus proposed in U.S. Pat. No. 5,268,790 is not so designed that the diffractive optical element provided therein contributes to a reduction in the number of the constituent lens elements of the entire lens system. Furthermore, the lens system proposed in Japanese Laid-open Patent Application No. H4-214516 for use in a stepper is unfit for use in an optical system that employs a halogen lamp, for the same reason as noted above for the objective lens system proposed in Japanese Laid-open Patent Application No. H6-242373 for use in an optical disk apparatus.